A plastisol in which polymer fine particles are dispersed in a plasticizer used as a medium has been used in various industrial fields such as automotive undercoats, auto body sealers, wall papers, carpet backings, floor materials, paints, and toys.
Conventionally, plastisols have mostly been vinyl chloride sols using vinyl chloride polymer particles. In recent years, a shift to acrylic sols in which (meth)acrylic polymer particles are used has been investigated from the consideration of global environment and the like. The acrylic sols have an advantage such that they do not generate toxic dioxins or halogenated hydrogen gases that are a cause of acid rain when products produced from the acrylic sols are incinerated because the acrylic sols do not contain halogen elements.
The shift from vinyl chloride sols to the acrylic sols is being positively advanced, in particular, in the uses of automotive undercoats and auto body sealers. This is because, in addition to the above environmental problem, there is a problem such that hydrogen chloride generated from vinyl chloride sols at a melting step of crushed steel in recycling automotives causes damage of equipment.
It is preferable that plastisols can stably maintain a sol state during storage, can have excellent gelation properties that causes fast gelation when the plastisols are heated, and can give coating films or shaped articles that have toughness and flexibility. As such acrylic sols, those containing a plasticizer and (meth)acrylic polymer particles, which are constructed from a core layer giving flexibility of coating films and a shell layer suppressing contact of a polymer in the core layer with the plasticizer and thus giving storage stability, and further compounded with a blocked isocyanate-based adhesive which gives coating films adhesive properties or strength, have been reported in Patent Document 1. Such acrylic sols can satisfy practically desired physical properties, however, they are liable to cause stringiness or sagging when coated if a large amount of the adhesive, which is a highly viscous liquid material, is compounded, and hence further improvement has been desired.
Further, there is a tendency that material costs of the acrylic sols become higher as compared with those of vinyl chloride sols. As main causes of this, the facts that raw material cost of (meth)acrylic polymer particles is high and that adhesives, curing agents, and anti-sagging agents tend to be compounded in large quantities can be listed.
Although various proposals have been offered to solve these problems, the most effective measure among them is the reduction of the content of the adhesives. However, when the content of the adhesives in the acrylic sols is reduced, not only adhesive properties but also chipping resistance or storage stability of shaped articles such as coating films shaped from the acrylic sols are liable to be lowered. Therefore, it has been desired to develop (meth)acrylic polymer particles by which a shaped article such as coating films having excellent chipping resistance and tensile strength can be obtained and whose storage stability is improved. However, (meth)acrylic polymer particles satisfying these demands have not been developed yet.
It has been reported in Patent Documents 2 and 3 that functional group-containing (meth)acrylic copolymer particles having a large particle diameter ranging from about several micrometers to 100 μm and a high glass transition temperature (Tg) are used as a resin component in order to improve storage stability of the acrylic sols. However, the storage stability of the acrylic sol composition described in Patent Document 2 is not sufficient in a practical industrial usage environment at 40° C. or above, though it is guaranteed at room temperature. In addition, as for the acrylic sol composition described in Patent Document 3, flexibility of the coating films obtained therefrom is not sufficient and chipping resistance of the coating films is liable to become insufficient. In addition, these acrylic sol compositions tend to have lowered anti-sagging property because particles having a large particle diameter are used and hence interactions among the particles are weak and hence viscosity is low.
Further, the acrylic sol composition containing a specific plasticizer and core-shell type polymer particles having a specific solubility parameter, which is referred to as Sp value, is proposed in Patent Document 4, however, when a general-purpose plasticizer is used, bleeding of the plasticizer or deficiency in storage stability occurs and chipping resistance of coating films to be obtained tends to be lowered. In addition, it is desired that the acrylic sol composition be improved to have storage stability in a practical industrial usage environment at 40° C. or above.
Further, the acrylic sol composition in which a monovinyl aromatic copolymer covered with a methacrylate polymer whose molecular weight is adjusted by a chain transfer agent, and further ionically crosslinked is used, is proposed in Patent Document 5. However, coating films shaped from the acrylic sol composition have insufficient chipping resistance.    Patent Document 1: International Publication No. WO 2000/001,748    Patent Document 2: Japanese Patent Application Laid-Open No. Sho 54-117,553    Patent Document 3: Japanese Patent Application Laid-Open No. Hei 7-102,147    Patent Document 4: International Publication No. WO 2003/004,568    Patent Document 4: Japanese Patent Application Laid-Open No. 2004-352,846